Corrosion and environmentaldegradation of bonded

composite repairG. Gkikas, A. Paipetis, A. Lekatou, N.M. Barkoula and D. Sioulas

Department of Materials Science & Engineering, University of Ioannina,Ioannina, Greece, and

B. Canflanca and S. FlorezINASMET-Tecnalia, Donostia-San Sebastian, Spain

Abstract

Purpose – Bonded composite patches are ideal for aircraft structural repair as they offer enhancedspecific properties, case-tailored performance and excellent corrosion resistance. Bonding minimizesinduced stress concentrations unlike mechanical fastening, whilst it seals the interface between thesubstrate and the patch and reduces the risk of fretting fatigue that could occur in the contact zone.The purpose of this paper is to assess the electrochemical corrosion performance and theenvironmentally induced mechanical degradation of aerospace epoxy adhesives when carbonnanotubes (CNTs) are used as an additive to the neat epoxy adhesive.

Design/methodology/approach – The galvanic effect between aluminium substrates and eitherplain or CNT enhanced carbon fibre composites, was measured using a standard galvanic cell. Also,rest potential measurements and cyclic polarizations were carried out for each of the studied systems.The effect of the CNT introduction to a carbon fiber reinforced plastic (CFRP) on the adhesionefficiency, before and after salt-spraying for 10, 20 and 30 days, was studied. The adhesion efficiencywas evaluated by the single lap joint test.

Findings – The corrosion behaviour of the system is polymer matrix type dependent. CNTintroduction to a CFRP may induce small scale localized degradation.

Originality/value – This paper fulfills an identified need to study how the shear strength and theresponse to galvanic corrosion are affected by epoxy resins modified by carbon nanotubes.

Keywords Shear strength, Corrosion resistance, Epoxy resins, Bonded repair,Carbon fiber reinforced plastics, Single lap joint, Carbon nanotubes, Galvanic corrosion, Salt spray

Paper type Research paper

1. IntroductionThe application of bonded composite patches as doublers to repair or reinforcedefective metallic structures is gaining recognition as a very effective and versatilerepair procedure for many types of damage. Various applications of this technologyinclude the repair of cracking, localized reinforcement after removal of corrosiondamage and reduction of fatigue strain (Baker, 1997). The bonded repair on the crackedmetallic structure allows for the restoration of strength and stiffness of the structure,whereas it hinders further crack growth by reducing the stress intensity factor.The application of bonded composite patches to fix defective secondary structures hasbecome a routine in recent years.

The current issue and full text archive of this journal is available at

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The authors would like to acknowledge the EU (IAPETUS project, Grant Agreement Number:ACP8-GA-2009-234333) for financial support.

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International Journal of StructuralIntegrity

Vol. 4 No. 1, 2013pp. 67-77

q Emerald Group Publishing Limited1757-9864

DOI 10.1108/17579861311303636

Although mechanically fastened patches are usually endorsed by aircraftmanufacturers, adhesively bonded patches have been reported to perform betterthan bolted patches, since the effect of the low shear strength of the adhesive layer issuperseded by the large contact area. Edge effects can be minimized by the tapering ofthe bonded patch. It is also clear that, irrespective of the static performance of therepaired component, fatigue loading may lead to crack initiation from fastener holes,which are typically loci of stress concentration. The better efficiency of bonded versusmechanically fastened repair has been demonstrated experimentally (Baker et al.,1981). Composite patched aluminum edged notched panels clearly exhibited superiorbehavior to fatigue loading, as the crack propagation under the patch was significantlydelayed when the doubler was bonded to the substrate (Backer et al., 2002).

Aircraft alloys are specially designed alloys to impart high strength and lightweight to aircrafts. However, especially the two most commonly used Al alloys AA2024 T-3 and AA 7075 T-6 (Buchheit et al., 1997; Ilevbare et al., 2000; Buchheit, 1995)are prone to corrosion. These two types of aluminum alloys are usually used in thefuselage, wings and spars of the aircrafts ( Jones and Hoeppner, 2009; Chlistovsky et al.,2007). The micro structural heterogeneity of these multi-phase alloys arises due toimpurities that are unavoidably introduced during melt processing. Unfortunately,such microstructure makes aluminum alloys susceptible to the formation of galvanicmicrocells and to localized corrosion during service. Also, these two alloys are amongthe most difficult to protect of all aluminum alloys (Miller et al., 1994; Reynolds et al.,1997; Janata et al., 1995). Corrosion is also of great importance in interfaces with othermaterials, such as reinforced polymers. It is well known that carbon fiber reinforcedplastics (CFRPs) induce galvanic corrosion when coupled with steel or aluminum inaggressive environments (Tavakkolizadeh and Saadatmanesh, 2001; Bellucci, 1992;Armstrong et al., 2006), and this inhibits their application for aircraft repair.

Previous research has shown that CFRPs using carbon nanotube (CNT) dopedmatrix material exhibit a spectacular improvement in fracture toughness under mode Iand mode II remote loading conditions (Kostopoulos et al., 2004a), whilst significantlyhigher damage tolerance properties and fatigue life extension, in comparison with theundoped matrix (Kostopoulos et al., 2004b, 2007, 2010). These enhanced properties canbe readily exploited in aircraft repair technologies. In a preliminary investigation,it was claimed that CNTs may alter the galvanic behavior of the both the CFRP andthe adhesive leading even to the reversal of the galvanic potential with aluminum(Gkikas et al., 2010).

The purpose of this study is to investigate whether the introduction of a smallamount of multi-wall CNTs into the matrix of CFRPs may lead to a decrease in theREDOX potential difference between CFRPR and Al-alloy and/or decrease in thegalvanic current of the cell CFRPR/Al-alloy, regardless of the nature of materialsemployed. This way, the CNT network may act as a galvanic corrosion barrier bybridging the carbon fiber/aluminum REDOX potential difference and reducing thegalvanic effect between these two system components. Furthermore, the susceptibilityof the CFRP doped by CNTs to localized degradation has been studied. Besideselectrochemical behaviour, the effect of the CNT introduction to a CFRP on theadhesion efficiency (by means of shear stress measurements), before and aftersalt-spraying, is evaluated.

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2. Experimental2.1 MaterialsMulti-wall CNTs provided by ARKEMA, France, were incorporated into epoxy(1wt% CNT/resin) using a calander – dissolver device. The incorporation of themulti-wall CNTs took place in INASMET – TECNALIA, Spain.

The resin system was composed of an epoxy system of two compounds, Epocast52 A and Epocast 52 B with mix ratio 100/41 parts by weight. The supplier of the resinsystem was HUNTSMAN ADVANCED MATERIALS, Switzerland.

The patches used in this study were five plies orientated in 08, carbon fabric Hexcel43280 (HRAS4) satin 5, 285 g/m2 impregnated with Epocast 52 (dopedwith CNT or not).

As a substrate, a typical aircraft alloy Al 2024 T3, provided by HELLENICAEROSPACE INDUSTRY,Greece,was employed.The thickness of theAl 2024 specimenwas 1.6mm and the surface treatment was Grit Blast/Silane & Primer (BR127) method.

2.2 Electrochemical studySmall rectangular coated coupons were cut with a diamond saw, in order to be subjectedto electrochemical testing. Ultrasonically cleaned coupons were encapsulated in PTFE,leaving a surface area of,1 cm2 to be exposed to aerated 3.5 percent NaCl, at 258C. Allthe electrochemical tests were performed using the Gill AC potensiostat/galvanostat byACM Instruments. A standard three electrode cell was employed, with Ag/AgCl(3.5M KCl, EAgCl ¼ ESHE – 200mV) as the reference electrode and a platinum gauge asthe counter electrode. The electrochemical cell was positioned in a Faraday case in orderto limit interference with the laboratory environment. Potentiodynamic polarizationtestswere carried out at a scan rate of 10mV/min. Reverse polarizationwas conducted tostudy the susceptibility of the systems to localized corrosion. The rest potential (Erest)was determined after 24 h of immersion in 3.5 percent NaCl, at room temperature (r.t.).Corrosion current densities were determined by Tafel extrapolation, as described inprevious work (Lekatou et al., 2010).

The galvanic current of the couples: anodized Al 2024 T3 – CFRP with neat epoxymatrix (Epocast), anodized Al 2024 T3 – CFRPwith doped 1 percent CNTs epoxymatrix (CNT_Epocast) versus time was continuously measured and recorded byelectrically connecting the couple constituents (PTFE masked specimens leaving equalsurface areas exposed to the electrolyte) through a zero resistance ammeter (Gill AC byACM Instruments of current range 10 pA-500mA). The Al-alloy was connected to theworking electrode 1 (WE1) input of the galvanostat. Should the galvanic current of thecouple receive positive values, then WE1 is anodic to WE2.

2.3 Shear behavior studyThe effect of CNTs on the adhesion efficiency and the environmental degradation wasstudied using the lap shear test, before and after salt spray exposure. The standard testmethod for lap shear adhesion for FRP bonding (ASTM: D 5868 – 95) and the standardpractice for operating salt spray (Fog) apparatus (ASTM: B117 – 11) were used for thelap shear testing and the environmental exposure, respectively. The dimensions of themanufactured coupons are shown in Figure 1.

The equipment used for testing the coupons was the mechanical testing machineINSTRON series 8801 and the video extensometer (Figure 2) for measuring the axialstrain. The displacement rate was 0.4mm/min.

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3. Results and discussion3.1 Potentiodynamic polarizationFigure 3 shows the cyclic polarization behaviour of anodized Al2024-T3 (Figure 3(a))and CFRPs with or without CNT doping (Figure 3(b)), in 3.5 percent NaCl, at roomtemperature. Table I lists the electrochemical values extracted from the polarizationcurves. Table I shows that doping the adhesive film with CNTs does not bridge the restpotential difference between the composite material and Al2024.

Figure 3(a) shows that the Al-alloy, even in the anodized state, is prone to localizedcorrosion. This is demonstrated by the:

. steep flattening of the gradient at overpotentials great than the breakawaypotential (Eb) that is sustained for more than three orders of magnitude currentincrease;

. the negative hysteresis loop (i.e. higher currents during reverse scanning,as compared with the respective currents during forward scanning); and

. lower anodic-to-cathodic transition potential as compared to the corrosionpotential revealing a more active surface.

Figure 1.Coupon for lap shear test

Figure 2.Video extensometer andINSTRON 8801

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It is well established that pitting susceptibility of Al-alloys is strongly associated withtheir intermetallic inclusions (Foley, 1986). Even in the case of anodizing, the anodicfilm contains flaws that constitute favoured sites for pit initiation. This is especiallythe case when the anodic film has deficiently been sealed in order not to loose itsability to anchor organic coatings (Altenpohl, 1998). Nevertheless, it should be notedthat anodizing has provided some protection against pitting, since otherwise, thecorrosion potential would be the same with the pitting potential, as occurs in aeratedenvironments (Stansbury and Buchanan, 2000), this is because the cathodic oxygen

Figure 3.Cyclic polarization curves

for the: (a) anodizedAl2024-T3 and (b) CFRPs

with or without CNTdoping, in 3.5 percent

NaCl, r.t

Al 2024 anodized

–2,000

–1,500

–1,000

–500

0

500

1,000

0.00001 0.0001 0.001 0.01 0.1 1 10 100

Current density (mA/cm2)

Pot

entia

l (m

V, A

g/A

gCl)

Al2024 forward

Al2024 reverse

Ecorr

Ea/c tr

Eb

–2,000

–1,500

–1,000

–500

0

500

1,000

1,500

0.000001 0.00001 0.0001 0.001 0.01 0.1 1Current density (mA/cm2)

(a)

(b)

Pot

entia

l (m

V, A

g/A

gCl)

Epocast forward

Epocast reverse

CNT_Epocast forward

CNT_Epocast reverse

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reduction, largely within the flawed regions, is sufficient to raise the corrosion potentialto the pitting potential. The small values of corrosion current density are typical of thealuminum alloys; although Al-alloys present quite low corrosion rates in aqueouschlorides, they suffer from pitting corrosion in these solutions.

Figure 3(b) shows the cyclic polarization behaviour of the Epocast systems. Bothsystems demonstrate a conductive behaviour due to the conductive nature of thecarbon fibres and CNTs. The Epocast system exhibits a general corrosion behaviour.The slightly higher current densities during reverse polarization may be explained bythe water adsorption that may preferentially occur at the fibre/matrix interfaces.Nevertheless, the same values of the corrosion potential and the anodic-to-cathodictransition potential and the small area within the negative loop indicate an essentiallyuniform process governed by the uniform distribution of the reinforcements and thereinforcement/matrix interfaces.

In the case of the CNT doped system, the shape of the forward anodic curve is morecomplicated and is characterized by abrupt changes in the gradients, indicatinglocalized phenomena. This consideration is enhanced by the negative loop hysteresisand the (slightly though) lower anodic-to-cathodic transition potential as compared tothe corrosion potential. The localized degradation is possibly associated with thepresence of CNTs. This possibility is further enhanced by the observation of similarphenomena during polarization of other types of neat epoxy doped with CNTs, whichhowever will be the subject of a future publication. Hence, it is indicated that weakmatrix/CNT interfaces may induce localized degradation of the epoxy leading to sharpcurrent increases.

3.2 Galvanic effectFigure 4 shows the galvanic current vs immersion time plots for the couples: anodizedAl 2024 T3 – CFRP with neat epoxy matrix (Epocast), anodized Al 2024 T3 – CFRPwith doped 1 percent CNTs epoxy matrix (CNT_Epocast).

The positive values of the current density show that the Al-alloy is anodic to thecomposites, in compatibility with the rest potential measurements (Table I). Therefore,regarding the systems CFRP/Al2024, should the Cl2 ions reach the alloy substrate itscorrosion will be accelerated, as the latter is anodic to its coating. This coatingarrangement is susceptible to detachment of the coating from its substrate, in case thatthe electrolyte reaches the interface (Guilemany et al., 2006).

In the case of the CFRP material with neat epoxy matrix, the galvanic current valuesafter ,8 h of immersion approach zero, which means that the ion movement is

MaterialErest, 24 h(mV)a

Ecorr

(mV)Eb

(mV)Ea/c/tr

(mV)Eb 2 Ecor

(mV)Ea/c/r 2 Ecor

(mV)icorr

(mA/cm2)ipit

(mA/cm2)

Epocast 2693 2603 2604 21 b

CNT_Epocast 2580 2303 100? 2327 680? 224 0.98Al2024-T3 2752 2794 2649 2858 149 264 0.002 0.009

Notes: aAll potential values are cited against the reference Ag/AgCl electrode; bcannot confidently bedetermined, since a linear portion for at least one order of magnitude current change could befound neither in the cathodic nor in the anodic part of the polarization curve; Ecorr – corrosion potential;Eb – breakawaypotential; Ea/c/tr – anodic-to-cathodic transition potential; icor – corrosion current density

Table I.Electrochemical valuesof the materials immersedin 3.5 percent NaCl, r.t.Erest, 24 h: open circuitpotential afterimmersion for 24 h

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neglectible and, consequently, no galvanic corrosion phenomena occur between thetwo materials. The initial fast decrease in current density indicates passivation ofthe Al-alloy. More accurately, the current oscillations indicate deposition of instablecorrosion products, pit formation on Al2024 and repassivation. In the case ofthe Al-CNT doped epoxy, the increasing trend of current density after ,20 h ofimmersion suggests a state towards stable pitting.

However, the most important observation is that the presence of the CNTs is notbeneficial, as it does not mediate the effects of the galvanic corrosion that lead to stressfailure and premature failure. This is in contrast with previous work (Gkikas et al.,2010), where the CNTs inclusion had reduced the galvanic effect between the substrateand the patch. Hence, it is inferred that the corrosion behaviour of these materials issystem (type of adhesive and hardener) dependent.

Nevertheless, a very interesting result is that the CNT incorporation in the CFRPresults in a notably nobler surface, regarding not only the corrosion potential and therest potential, but also the generated currents; the polarization curve of the CNT-dopedCFRP is located at higher potentials and lower current densities, in comparison withthe CNT-free CFRP (Figure 3(b)).

3.3 Effect of CNTs on the adhesion efficiency and the environmental degradationFigure 5(a) shows that the addition of CNTs did not significantly alter the adhesionefficiency. Although a slight decrease in lap shear strength may be noted, thedifferences are within the standard deviation of the measurements. However, it shouldbe noted that the properties of the resulting nanomodified polymer are very dependenton parameters such as the dispersion and the curing process. Although it is not withinthe purpose of this study, the optimization of these parameters would be expected toresult to better adhesion properties, as it has already been demonstrated for typicalCNT reinforced epoxy systems (Montazeri et al., 2011).

For the specimens whose structural adhesive is undoped, a substantial decrease inthe shear strength occurred after exposure to the salt spray environment for over

Figure 4.Galvanic current vs time

for the couples:Al 2024-carbon fibre

reinforced epoxy,Al 2024-carbon fibre

reinforced epoxy dopedwith 1 percent CNTs,

in 3.5 percent NaCl, at r.t

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Figure 5.Shear stress for allcoupons before and aftersalt spray exposure

20

18

16

14

12

8

10Sh

ear

stre

es (

MPa

)Sh

ear

stre

es (

MPa

)Sh

ear

stre

es (

MPa

)

6

4

2

0

2018161412

810

6420

20

18

16

14

12

8

10

6

4

2

0

AI-epocast (0d exp.)

AI-epocast(0d exp.)

AI-epocast(10d exp.)

AI-epocast(20d exp.)

AI-epocast(30d exp.)

AI-CNT_epocast(0d exp.)

AI-CNT_epocast(10d exp.)

AI-CNT_epocast(20d exp.)

AI-CNT_epocast(30d exp.)

AI-CNT_epocast (0d exp.)

Aluminum substrate - Epocast

Aluminum substrate - CNT_epocast

(a)

(b)

(c)

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20 days (Figure 5(b)). Regarding the specimens of which the structural adhesive hadbeen modified by nano-tubes, a substantial decrease in their shear strength was notedin the 10-20 day span of exposure (Figure 5(c)). Hence, once more, in compatibility withthe potentiodynamic polarization and galvanic effect studies, the introduction of weakCNT/CFRP interfaces in the CFRP is suggested. The access of the electrolyte to thesubstrate is accelerated due to the existence of these weak interfaces resulting in fastershear strength decrease, as compared to the CNT-free CFRP.

4. ConclusionsRegarding the system anodized Al2024-carbon fibre reinforced epoxy, which wasstudied in this work, the incorporation of CNTs into the matrix had no significant effecton reducing the galvanic effect between the adhesive and aluminium. This is incontrast with previous work (Gkikas et al., 2010), leading to the conclusion that thecorrosion behavior of these materials is system (type of the adhesive and the hardener)dependent.

As regards the shear stress, CNT doping of the CFRP matrix does not alter the lapshear strengthwithin the experimental error. Upon salt spraying, the CNTdoped systemalso showed faster shear strength decrease as compared to the Al substrate-CFRPundoped matrix. However, as should be noted, the effective dispersion and the cuttingdegree of the nanomodified sytem is very dependent on processing parameters andmay be optimized, as it has already been demonstrated for typical nanoreinforcedepoxies. The current study verifies that the presence of the CNTs alters the galvanicbehavior of the system. However, in the case of the studied Epocast/carbon fibrecomposite, it leads to increased corrosion current and susceptibility to pitting corrosion,as cyclic polarization tests have indicated. For such material behavior, repairtechnologies that incorporate bi-layer patches, where the intermediate layer will be theunmodified CFRP (which is compatible with the Al-substrate in terms of galvaniccorrosion, as Figure 4 suggests) and the over-layer will be the CNT-doped CFRP, couldbe considered.

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Corresponding authorA. Paipetis can be contacted at: paipetis@cc.uoi.gr

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